1. Field of the Invention
This invention relates to a photo-conductor layer for constituting a radiation imaging panel, which is appropriate for use in a radiation imaging apparatus, such as an X-ray imaging apparatus.
2. Description of the Related Art
There have heretofore been proposed X-ray imaging panels designed for use in a medical X-ray image recording operation, such that a radiation dose delivered to an object during the medical X-ray image recording operation may be kept small, and such that the image quality of an image and its capability of serving as an effective tool in, particularly, the efficient and accurate diagnosis of an illness may be enhanced. With the proposed X-ray imaging panels, a photo-conductor layer sensitive to X-rays is employed as a photosensitive material. The photo-conductor layer is exposed to X-rays carrying X-ray image information, and an electrostatic latent image is thereby formed on the photo-conductor layer. Thereafter, the electrostatic latent image, which has been formed on the photo-conductor layer, is read out by use of light or a plurality of electrodes. The techniques utilizing the X-ray imaging panels have advantages over the known photo-fluorography utilizing TV image pickup tubes in that an image is capable of being obtained with a high resolution.
Specifically, when X-rays are irradiated to a charge forming layer located in the X-ray imaging panel, electric charges corresponding to X-ray energy are formed in the charge forming layer. The thus formed electric charges are read out as an electric signal. The photo-conductor layer described above acts as the charge forming layer. As the material for the photo-conductor layer, amorphous selenium (a-Se) has heretofore been used. However, ordinarily, amorphous selenium has the problems in that it is necessary for the layer thickness of the photo-conductor layer to be set to be large (e.g., at least 500 μm) because of a low radiation absorptivity of amorphous selenium.
However, in cases where the layer thickness of the photo-conductor layer is set to be large, the electric charge collecting efficiency becomes low, and it becomes necessary for a high voltage to be applied across the photo-conductor layer. However, in such cases, the problems occur in that electric charges due to a dark current are apt to occur, the contrast in a low dose region becomes low, and device deterioration is apt to occur. Also, the problems occur in that noise (structure noise) occurring due to a variation in selenium in the thickness direction is apt to be collected. Further, ordinarily, the photo-conductor layer is formed by use of a vacuum evaporation technique. However, in cases where the vacuum evaporation technique is employed, considerable time is required to grow the photo-conductor layer up to the large layer thickness described above, and management of the growth of the photo-conductor layer is not easy to perform. As a result, the production cost of the photo-conductor layer is not capable of being kept low, and the cost of the X-ray imaging panel is not capable of being kept low.
Because of the problems described above, it has been studied to utilize materials for the photo-conductor layer other than amorphous selenium. By way of example, as a substance for constituting the photo-conductor layer, there has been proposed a bismuth oxide type of a composite oxide. The proposed bismuth oxide type of the composite oxide may be represented by the formula BixMOy, in which M represents at least one kind of element selected from the group consisting of Ge, Si, and Ti, x represents a number satisfying the condition 10≦x≦14, and y represents the stoichiometric oxygen atom number in accordance with M and x. The proposed bismuth oxide type of the composite oxide is described in, for example, each of Japanese Unexamined Patent Publication Nos. 11(1999)-237478 and 2000-249769. With the proposed bismuth oxide type of the composite oxide, it is expected that the efficiency, with which the X-rays are converted into the electric charges, will be capable of being enhanced.
However, in each of Japanese Unexamined Patent Publication Nos. 11(1999)-237478 and 2000-249769 described above, as a technique for forming the photo-conductor layer, a technique is employed, wherein a sol or a gel having been obtained from hydrolysis of a bismuth alkoxide and a metal alkoxide is subjected to sintering processing, and wherein the resulting sintered material is subjected to dispersion and coating. However, with the aforesaid technique for forming the photo-conductor layer, limitation is imposed upon a packing density of the photo-conductor substance in the photo-conductor layer which is capable of being formed with the coating. Also, the problems occur in that a binder, which is contained in the photo-conductor layer having been formed with the coating, has a large effect of obstructing the movements of the generated electric charges, electric noise becomes large, and therefore graininess characteristics of the obtained image are not capable of being kept good.
A process for producing a Bi12MO20 sintered material by use of a solid phase technique is described in, for example, each of J. Am. Ceram. Soc. 84, 2900 (2001) to “Processing and Dielectric Properties of Sillenite Compounds Bi12MO20-8 (M=Si, Ge, Ti, Pb, Mn, B1/2P1/2)”, by M. Valant and D. Suvorov, J. Am. Ceram. Soc., Vol. 84, pp. 2900-2904, 2001 and Ceramic Bulletin 58, 613 (1979)to “Sintering of an X-Ray Photosensitive Ceramic: Bi12GeO20”, by A. Morell and A. Hermosin, Ceramic Bulletin Vol. 58, pp. 613-615, 1979. The Bi12MO20 sintered material, which is obtained with the sintering processing, has the advantages over the photo-conductor layer, which is formed with the coating, in that the packing density of the photo-conductor substance in the photo-conductor layer is capable of being enhanced because of a dense structure, and therefore the sensitivity of the photo-conductor layer is capable of being enhanced. However, with the process for producing a Bi12MO20 sintered material by use of a solid phase technique, which process is described in each of J. Am. Ceram. Soc. 84, 2900 (2001) and Ceramic Bulletin 58, 613 (1979), though the packing density of the photo-conductor substance in the photo-conductor layer is capable of being kept higher than the packing density of the photo-conductor substance in the photo-conductor layer having been formed with the coating, the sensitivity is not always capable of being enhanced to an expected level.
In the field of ceramic materials, it has been found that the state of oxidation of an oxide is determined by an oxygen partial pressure, and that the oxygen concentration in the atmosphere is determined by a ratio of a CO gas to a CO2 gas. (The aforesaid findings are described in, for example, “Handbook of Ceramics”, Second Edition, edited by The Ceramic Society of Japan, pp. 423-426.) Also, in “Handbook of Ceramics” there is a description concerning a technique for controlling the oxygen partial pressure by use of a gas, which contains an N2 gas as a principal constituent and which contains at least one kind of gas selected from the group consisting of H2, H2O, CO2, and CO, in a sintering step for a laminated ceramic condenser.
The inventors have conducted extensive research concerning the conditions for the production of a Bi12MO20 sintered material and found that a Bi12MO20 sintered material, which is free from defects obstructing the movements of electric charges, is capable of being obtained, and that a radiation imaging panel having a high sensitivity is capable of being obtained by use of the aforesaid defect-free Bi12MO20 sintered material for the photo-conductor layer.